Improve consistency for Assignee and Assignor's permissions: give them 'Delete objects' permission, as they already have 'Add portal content' and 'Modify portal content'.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31677 20353a03-c40f-0410-a6d1-a30d3c3de9de